Method of manufacturing spatial light modulator and electronic device employing it

ABSTRACT

A spatial light modulator is constructed from a conductive silicon mirror substrate and a glass electrode substrate including sodium, anode-bonded together. The silicon mirror substrate has micromirrors arranged in a matrix, torsion bars coupling these micromirrors in the x-direction, and a frame coupled to both ends of the torsion bars. A glass electrode substrate has a central depression, a rim around the periphery thereof, pillars projecting from within the depression, and electrodes and wiring driving micromirrors formed within the depression in an inclining manner. Both ends of the torsion bars are bonded to the rim of the frame portion, and intermediate portions of the torsion bars are bonded to the pillars. Both ends of the torsion bars are cut away from the frame portion during dicing.

CONTINUING APPLICATION DATA

This application is a divisional of U.S. patent application Ser. No.08/756,463, filed Nov. 26, 1996, now U.S. Pat. No. 5,999,306 thecontents of which are incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing spatial lightmodulator and electronic device employing it.

2. Description of the Related Art

This type of spatial light modulator is disclosed, for example JapanesePatent Application Laid-Open Nos. 4-230722, 5-188308, and 5-196880. Animproved form of these devices is also described in the March 1994 issueof "Nikkei Microdevice" as a "DMD" (Digital Micromirror Device).

This DMD has, as shown in FIG. 22, a three-layer construction comprisingan upper layer 800, an intermediate layer 810, and a lower layer 830.

The upper layer 800 comprises a mirror 802 and a mirror support post 804joined to the center of the lower surface of the mirror 802. Inconnection with the fabrication process of the mirror 802, in a positionopposite to the mirror support post 804 is formed a depression 806.

The intermediate layer 810 has a mirror support plate 812 which iscoupled to the mirror support post 804, and which is supported atopposite ends by hinges 814 so as to be able to be driven in aninclining manner. To provide the space for this mirror support plate 812for driving in an inclining manner, the hinges 814 have on their lowersides hinge support posts 816.

The intermediate layer 810 further is provided with first and secondaddress electrodes 818 and 820 on opposing sides of the hinges 814, eachsupported by electrode supporting posts 826. Furthermore, outside thisare provided a first mirror contact electrode 822 and second mirrorcontact electrode 824, each supported by electrode supporting posts 826.

The lower layer 830 comprises four electrodes 832a to 832d coupled tothe electrode supporting posts 826 of the first and second addresselectrodes 818 and 820, and a common electrode 834 coupled to the firstand second mirror contact electrodes 822 and 824.

This DMD, as shown in FIG. 23, has a bias voltage Va applied to themirror 802 and the first and second mirror contact electrodes 822 and824. Then when for example a negative voltage is applied to the firstaddress electrode 818, and a positive voltage is applied to the secondaddress electrode 820, a Coulomb force acts between the mirror 802 andthe first address electrode 818, and the mirror 802 is driven to aninclined position as shown by the dot-dash line in FIG. 23. By reversingthe polarity of the voltage applied to the first and second addresselectrodes 818 and 820, an inclined position as shown by thedot-dot-dash line in FIG. 23 can be established.

The inclined position of the mirror 802 shown by a dot-dash line in FIG.23 is taken to be the "ON" position in which light is reflected toward acertain position, and the inclined position shown by a dot-dot-dash lineis taken to be the "OFF" position in which light is reflected in adifferent direction. By varying the time between switches, a256-gradation display can be obtained.

The DMD shown in FIG. 22 is hypothetically manufacturable by afabrication process as shown in FIGS. 24A to 24H and FIGS. 25A to 25F.FIGS. 24A to 24H show the steps in the formation of the intermediatelayer 810 on an already formed lower layer 830, and FIGS. 25A to 25Fshow the steps in the formation of the upper layer 800 on theintermediate layer 810, and the formation of the interlayer spaces.

As shown in FIG. 24A, a substrate 840 on which an SRAM (static randomaccess memory) is formed as the lower layer 830 is provided. Next, asshown in FIG. 24B, a resist 842 is coated on this substrate 840, and inthe stage shown in FIG. 24C a pattern corresponding to the hinge supportposts 816 and electrode supporting posts 826 is formed.

As shown in FIG. 24D, an aluminum (Al) film is formed by vapordeposition over the surface of the resist 842 and trench portion, andthen further as shown in FIG. 24E an aluminum oxide film 846 is formedover the surface.

Further after vapor deposition of an aluminum film 848 as shown in FIG.24F, as shown in FIG. 24G a resist 850 is applied in a pattern.Thereafter, as shown in FIG. 24H, the aluminum film 848 is etched,whereby mirror support plate 812, hinges 814, and hinge support posts816 are formed.

By the process shown in FIGS. 25A to 25F, the upper layer 800 shown inFIG. 22 is formed. For this purpose, as shown in FIG. 25A a resist 852is applied thickly, and is formed in a pattern as shown in FIG. 25B.Further, an aluminum film 854 is formed by vapor deposition, and afteran aluminum oxide film 856 is formed over a part of the surface thereof,the extremities of the aluminum film 854 are removed by etching, wherebythe mirror 802 and mirror support post 804 are formed. (See FIGS. 25C to25E.)

Finally, as shown in FIG. 25F, by removing the resist 842 and 852, aspace between the upper layer 800 and intermediate layer 810 is formed,and moreover a space between the intermediate layer 810 and lower layer830 is formed.

However, in the above process, there is the problem that the DMD cannotbe obtained with a high yield. One reason for this is that the factordetermining the angle of inclination of the mirror 802, that is, thedistance between the lower surface of the mirror 802 and the mirrorcontact electrodes 822 and 824 depends on the thickness of the resist852 in the resist step shown in FIG. 25A.

In general, such a resist is formed by the spin coating method, andwhile it is difficult in itself to improve the uniformity of a resistlayer thickness, when the spin coating method is used it is extremelydifficult to make the resist 852 of a uniform thickness.

Moreover, in the conventional spin coating method, the larger thesurface area of the wafer, the more difficult it is to ensure uniformitywithin the area of the resist film, and further to make the thickness ofthe resist film constant is for a large diameter semiconductor waferalmost impossible. Thus, it is difficult to form a plurality of devicessimultaneously from a single semiconductor wafer, and the throughput isreduced.

In addition to the above problems, a further one is that in the stage ofremoving the resist shown in FIG. 25F, it is difficult to completelyremove the resist from the furthest recesses of the underside of themirror 802 and hinges 814. If foreign objects are thus left behind, themirror 802 and address electrodes 818 and 820 may short-circuit, or theinclination of the mirror may be obstructed, or the mirror contactelectrodes 822 and 824 and address electrodes 818 and 820 mayshort-circuit.

Another problem with the above described construction of a DMD is thatthe depression 806 is formed in the center region of the mirror 802. Inthe aluminum apor deposition step of FIG. 25C, when aluminum is vapordeposited in the trench portion, the position opposing this trench isinevitably concave, and the forming of the depression 806 cannot beprevented.

In this three-layer DMD, since the hinges 814 are not in the same planeas the mirror 802, the exposed surface area of the mirror 802 isincreased, and the benefit is obtained of an increased light utilizationratio.

However, since the depression 806 is formed in the center of the largearea mirror 802, with this depression 806 in the line of a powerful beamof light, the light utilization ratio is actually reduced by the diffusereflection. Alternatively, the diffusely reflected light may be input asinformation pertaining to another pixel, resulting in the problem ofreduced image quality. Moreover, even if the side walls of thedepression 806 are processed so as to be vertical, the area which isoptically effective is reduced.

A further problem is that the above described spatial light modulator isformed on a substrate 840 on which an SRAM is formed, and the overallyield is the product of the yield of the SRAM and the yield of thespatial light modulator, which is thus considerably low.

Another prior art is the spatial light modulator described in Petersen,"Silicon as a Mechanical Material--Proceedings of the IEEE, Vol. 70, No.5, May 1982, in FIGS. 39, 40 and 41 on pages 442 and 448. In order tofabricate this, a silicon substrate which has been cut and ground onboth sides is used, and a micromirror is formed on this siliconsubstrate by photolithography and etching processes. The siliconsubstrate on which this micromirror is formed and a glass plate on whicha metal electrode film is formed are bonded by the anode bonding method,and a spatial light modulator thus manufactured.

By this method, however, in order to cut and grind the silicon substrateon both sides, and thus determine the substrate thickness, it is notpossible to obtain a thickness less than 200 mm. This is becausegrinding to a thickness less than this leads to breakage of the siliconsubstrate. The thickness of the micromirror is therefore at least 200mm, and the inertial moment due to this heavy mass is thus great, makingrapid response and high resolution display impossible.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a spatial lightmodulator equipped with micromirrors which can be fabricated with a highyield, a method for manufacturing the same, and an electronic deviceemploying the spatial light modulator.

Another object of the present invention is to provide a spatial lightmodulator equipped with micromirrors which allows accurate control ofspatial light modulation without the generation of diffuse reflection onthe surface of the micromirrors, a method for manufacturing the same,and an electronic device employing the spatial light modulator.

Yet a further object of the present invention is to provide a spatiallight modulator for which the yield is high, and for which amoving-picture gradation display is easy, a method for manufacturing thesame, and an electronic device employing the spatial light modulator.

The method of the present invention pertains to fabricating a spatiallight modulator having micromirrors, by bonding together a conductivesilicon mirror substrate and an electrode substrate. The conductivesilicon mirror substrate has a plurality of micromirrors arranged in oneof a line and in matrix and a torsion bar coupling the micromirrors inone direction, and a reflective layer is formed at least on one surfaceof the micromirrors.

The electrode substrate has a depression in a central region, a rimaround the periphery thereof, a set of electrodes having conductinglayers disposed within the depression in positions corresponding to themicromirrors, and driving the micromirrors in an inclining manner bymeans of a Coulomb force, and pillars projecting from the depression inpositions corresponding to the interval between two of the micromirrorsadjacent in the one direction.

In the step of bonding together the conductive silicon mirror substrateand the electrode substrate, at least intermediate portions of thetorsion bar on the silicon mirror substrate are opposite to the pillarsof the electrode substrate.

In this way, if the depression in the glass electrode substrate ispreviously formed with a depth of high accuracy, the deflection angle ofthe micromirrors can be determined accurately from lot to lot.Furthermore, the reflective layer formed on the surface of themicromirrors can be made uniform, and a surface with no diffusereflection can be formed.

In particular, when for example the electrode substrate is employed aglass electrode substrate including an alkali metal such as sodium, thesubstrates can be bonded using anode bonding. This means that noadhesive layer is required between the substrates, and thus thedeflection angle of the micromirrors can be determined accurately fromlot to lot.

It should be noted that the bonding method is not, however, restrictedto anode bonding, and direct bonding or diffusion bonding can also beused, and more detailed description of the bonding method is givenbelow. Furthermore, if heat is applied in the bonding process, thematerial of the electrode substrate should preferably be a material witha coefficient of thermal expansion close to that of silicon.

On the silicon mirror substrate, a frame portion to which both ends ofthe torsion bar are coupled may be formed. In this case, the frameportion and both ends of the torsion bar are bonded to the rim of theelectrode substrate. In addition, after this bonding, a step of cuttingaway both ends of the torsion bar from the frame portion.

By this means, the mutual positional relationship of a plurality oftorsion bars is maintained by the frame, and therefore without preciselypositioning each torsion bar the mutual positional relationship betweenthem can be maintained during bonding to the electrode substrate.

The process of step of fabricating the silicon mirror substrate maycomprise the steps of:

doping a silicon substrate with impurities to form a doped layer;

patterning a first mask for forming a window on one surface of saidsilicon substrate and a second mask for forming said plurality ofmicromirrors and said at least one torsion bar on the other surface ofsaid silicon substrate;

etching said silicon substrate until said doped layer is exposed usingsaid first mask;

a step of etching said doped layer using said second mask;

removing said first and second masks and forming said plurality ofmicromirrors and said at least one torsion bar from said doped layer;and

forming said reflective layers on one surface of said micromirrors ofsaid doped layer.

In another aspect of the present invention, before the silicon mirrorsubstrate is completed, while in the form of a silicon substrate, it isbonded with the electrode substrate.

The electrode substrate has a depression in a central region, a rimaround the periphery thereof, a set of electrodes having conductinglayers disposed within the depression in positions corresponding to themicromirrors, and driving the micromirrors in an inclining manner bymeans of a Coulomb force, and pillars projecting from the depression inpositions corresponding to the interval between two of the micromirrorsadjacent in one direction.

Bonded to this is a silicon substrate on one surface of which is formeda doped layer doped with impurities. At this time, at least the pillarsof the electrode substrate and the doped layer are opposite and bonded.

In this step, since the step is carried out before the micromirrors areformed, positioning for the bonding operation is simple.

Thereafter, the silicon substrate is etched to remove same, leaving thedoped layer, and a reflective layer is formed on the surface of thedoped layer.

Thereafter, the doped layer is etched. At this point a plurality ofmicromirrors are formed in positions opposite the set of electrodes. Thetorsion bar is formed coupling the micromirrors in one direction, bondedto the pillars at positions intermediate between two of the micromirrorsadjacent in that direction.

During the patterning for this etching step, when the positionalrelationship with the set of electrodes already formed on the electrodesubstrate is considered, with the accuracy of a photolithographyprocess, the micromirrors can be formed with high precision.

Using this method, the substrate positioning for bonding is easy, andmoreover since the micromirrors and so forth can be fabricated afterbonding, the method can be applied to high density layout of themicromirrors.

It should be noted that in the above method anode bonding can beadopted, or a frame portion can be formed on the silicon mirrorsubstrate.

When the micromirrors are arranged in a high density layout, theelectrode substrate may be formed of a transparent glass electrodesubstrate, and then the position of the pattern of the set of electrodesbe observed from the side of the glass electrode substrate, and usingthis pattern position as a reference, the mask pattern alignment for theetching of the silicon electrode substrate carried out.

In the method inventions above, if the impurity concentration of thedoped layer is at least 1×10¹⁸ atm/cm³, then during the etching of thesilicon substrate the doped layer can be used to function as an etchingstop layer.

The method of fabricating the glass electrode substrate may include thesteps of:

masking positions corresponding to said rim and said pillars and etchinga glass substrate including an alkalimetal to form said depression of apredetermined depth; and

forming said sets of electrodes on the base of said depression. In thiscase the depth of the depressions which affects the deflection angle ofthe micromirrors, depends on the etching conditions.

The set of electrodes may be formed as a set of transparent electrodesof for example ITO (indium tin oxide), and before the bonding, there maybe a step of inspecting the presence of foreign objects between theglass electrode substrate and the silicon mirror substrate from the sideof the glass electrode substrate. If this inspection is carried outbefore the bonding, the yield is increased and when carried out afterthe bonding, the ingress of foreign objects which is a cause ofdefective products can be detected easily.

There may be an additional step of bonding a transparent cover plate onthe silicon mirror substrate so as to cover the silicon mirror substrateand in a position non-interference with the micromirrors driven in aninclining manner.

By means of this transparent cover plate, the ingress of foreign objectswhich would impede the driving in an inclining manner of themicromirrors can be prevented, and the element protected.

The device of the present invention has a conductive silicon mirrorsubstrate doped with impurities and an electrode substrate bondedintegrally, wherein the silicon mirror substrate, comprises:

a plurality of micromirrors arranged in one of a line and matrix andhaving reflective layers formed on one surface; and

a torsion bar coupling said micromirrors in one direction;

at least one said electrode substrate comprises:

a depression in a central region thereof;

a rim around the periphery thereof;

sets of electrodes formed within said depression in positionscorresponding to said micromirrors and driving said micromirrors in aninclining manner by means of a coulomb force; and

pillars projecting from said depression in positions corresponding to aninterval between two of said micromirrors adjacent in said onedirection; and wherein

at least intermediate portions of said at least one torsion bar on saidsilicon mirror substrate are opposite said pillars of said electrodesubstrate, and said silicon mirror substrate and said electrodesubstrate are bonded. This bonding may be carried out by for exampledirect bonding or eutectic bonding.

The entire surface of the reflective layer formed on the micromirrors isformed as a flat surface. It can therefore reflect impinging light withan angle of reflection equal to the angle of incidence.

The set of electrodes is preferably formed as a set of transparentelectrodes of for example ITO (indium tin oxide). By looking through theglass electrode substrate, the ingress of foreign objects between theset of electrodes and the micromirrors, which would result in adefective product, can easily be detected.

Where the micromirrors are opposite the set of electrodes an insulatingfilm may be formed so that in when foreign objects ingress between themicromirrors and the set of electrodes, the serious problem of ashort-circuit can be avoided.

The surface of the set of electrodes where the electrodes are oppositethe insulating film formed on the micromirrors, may further be formed tobe rough. The contact area between the insulating film and the set ofelectrodes is reduced, and the micromirrors sticking to the set ofelectrodes caused by static charge on the insulating film can beprevented.

The surface roughness is preferably provided by forming on the surfaceof the set of electrodes projections of height at least 200 Angstroms.In this way adequate roughness can be assured to prevent stickingbetween the micromirrors and the set of electrodes. It should be notedthat if the gap between the micromirrors and the set of electrodes whenthe micromirrors and set of electrodes are parallel is G, then the upperlimit to the height of these projections should be not more than G/3.This assures the minimum deflection angle of the micromirrors requiredfor functional reasons.

To prevent sticking of the micromirrors, an insulating projection may beformed on the insulating film and at a position displaced from thetorsion bar.

As another method of preventing sticking of the micromirrors, aninsulating stopper may be formed. The insulating stopper projects fromthe base of the depression of the glass electrode substrate to a heightless than the height of the rim and the pillars, and abuts themicromirrors when driven in an inclining manner, in order to determinethe deflection angle.

Using the spatial light modulator of the present invention, variouselectronic devices can be constructed.

For example, a projector can be constructed from a projection lamp, aspatial light modulator which reflects light emitted by the projectionlamp modulated for each pixel by driving in an inclining manner each ofa plurality of micromirrors arranged one per pixel, and a projectionlens which projects an enlarged image of the light reflected from thespatial light modulator on a screen.

An electronic photography apparatus can be constructed from aphotosensitive drum on which a latent image is to be formed, a spatiallight modulator which reflects light sequentially, and emits reflectedlight modulated while scanning in one direction toward thephotosensitive drum to form a latent image by driving in an incliningmanner each of a plurality of micromirrors arranged in an array, adeveloping device developing the latent image formed on thephotosensitive drum, and a transfer device transferring the image on thephotosensitive drum to a recording medium.

Further, an optical switching device can be constructed from a pluralityof induction coils capable of generating desirable induction voltages, aspatial light modulator, and a wiring pattern connecting the inductioncoils and a set of electrodes of the spatial light modulator, aplurality of the micromirrors are each driven in an inclining manner,and a desirable optical signal is generated by light reflected from themicromirrors based on the induction voltages generated by each of theinduction coils.

In an exposure device which irradiates an exposure target with lightfrom a light source through an interposed mask to expose the exposuretarget, a spatial light modulator may be provided to reflect the lightfrom the light source from individual micromirrors, thus irradiating theexposure target with modulated light.

In this way, it is possible to record ID information such as a lotnumber using an exposure process on an exposure target such as asemiconductor wafer.

Another aspect of the spatial light modulator of the present invention,comprising:

a glass substrate on which at least one conductive torsion bar couplinga plurality of conductive micromirrors in one direction is supported bypillars, and on which a conductive frame portion fixing both ends ofsaid at least one torsion bar is formed; and

a circuit substrate on which a plurality of pairs of electrodes oppositeeach of said micromirrors and a circuit element energizing saidplurality of pairs of electrodes are formed;

and wherein said frame portion of said glass substrate and said circuitsubstrate are bonded.

In this way, the micromirrors and the circuit substrate can befabricated separately, and foreign objects can also be inspectedseparately, as a result of which the yield can be increased. Moreover,the region in which the micromirrors are disposed is covered by theglass substrate, frame portion, and circuit substrate.

The micromirrors and torsion bar may be formed from silicon or a metal.

The method of fabricating the micromirrors of silicon, comprises:

(a) forming on a glass substrate a depression in a central regionthereof, a rim surrounding said depression, and pillars formed toproject from said depression;

(b) diffusing impurities into one surface of a silicon substrate to apredetermined depth;

(c) further diffusing impurities into a predetermined portion of saidone surface of said silicon substrate to a predetermined depth to forman impurity diffusion surface;

(d) forming an optically reflective film on said impurity diffusionsurface of said silicon substrate;

(e) bonding said impurity diffusion surface and said rim of said glasssubstrate, to form a silicon-glass bonded substrate;

(f) wet-etching said silicon-glass bonded substrate to make said siliconsubstrate into a thin film;

(g) dry-etching said silicon substrate of said thin film to form aplurality of micromirrors, a torsion bar coupling and supporting thesame, and a frame portion fixing both ends of said torsion bar; and

(h) bonding to said frame portion of said silicon-glass bonded substratea silicon circuit substrate provided with a plurality of pairs ofelectrodes for driving said plurality of micromirrors, and circuitelements applying a drive voltage to said electrodes.

On the other hand, the method of fabricating the micromirrors of ametal, comprises:

(a) forming a first resist pattern on a glass substrate to form pillarsin a central region thereof and a first rim on the periphery thereof;

(b) forming a first metal film on said glass substrate and said firstresist pattern;

(c) forming on said first metal film a second resist pattern to formmicromirrors and a torsion bar;

(d) etching said first metal film using said second resist pattern;

(e) removing said second resist pattern;

(f) forming a third resist pattern in a region excluding a surface ofsaid first rim;

(g) forming a second metal film on said first rim and said third resistpattern;

(h) forming a fourth resist pattern on said second metal film and inposition opposite said first rim;

(i) etching said second metal film using said fourth resist pattern, andextending said first rim to form a second rim;

(j) removing said first, third, and fourth resist patterns; and

(k) bonding a silicon circuit substrate provided with circuit elementsfor driving said micromirrors and said second rim of said glasssubstrate.

In either of the methods, the circuit substrate and the glass substratemay be subjected to diffusion bonding or bonded using a conductiveadhesive.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings wherein like reference symbols refer to like parts.

FIG. 1 is a perspective exploded assembly view of a spatial lightmodulator of a first embodiment according to the present invention;

FIG. 2 is an enlarged perspective view of a micromirror used in thedevice shown in FIG. 1;

FIG. 3A is a plan view of a micromirror, and FIG. 3B is a side elevationof a micromirror;

FIGS. 4A to 4I are each schematic sectional drawings illustratingfabrication steps of a silicon mirror substrate of the spatial lightmodulator shown in FIG. 1;

FIGS. 5A to 5G are each schematic sectional drawings illustratingfabrication steps of a glass electrode substrate of the spatial lightmodulator shown in FIG. 1;

FIG. 6 is a schematic drawing illustrating the anode bonding process ofFIG. 4I;

FIG. 7A is a schematic drawing illustrating the spatial light modulatorsshown in FIG. 1 being fabricated on a single wafer, and FIG. 7B is aschematic perspective view of a single spatial light modulator separatedfrom the wafer;

FIG. 8 is a perspective exploded assembly view of a spatial lightmodulator of a second embodiment according to the present invention;

FIG. 9 is a perspective view of the spatial light modulator of thesecond embodiment;

FIGS. 10A to 10H are each schematic sectional drawings illustratingfabrication steps of the device shown in FIG. 8;

FIGS. 11A and 11B are each schematic sectional drawings illustrating athird embodiment of the present invention in which an insulating film isformed on the reverse side of a micromirror;

FIG. 12 is a schematic sectional drawing illustrating a variant of thethird embodiment in which a micropyramid is formed at the extremities ofthe insulating film formed on the reverse side of the micromirror;

FIGS. 13A to 13C are each schematic sectional drawings illustratingformation steps of the micropyramid shown in FIG. 12;

FIG. 14 is a schematic drawing illustrating a variant of the thirdembodiment in which an insulating stopper is provided on the glasselectrode substrate;

FIG. 15 is a schematic drawing of a fourth embodiment of the presentinvention, constituted by a projector provided with a single spatiallight modulator;

FIG. 16 is a schematic drawing of a variant of the fourth embodiment ofthe present invention, constituted by a projector provided with a doublespatial light modulator;

FIG. 17 is a schematic drawing of a variant of the fourth embodiment ofthe present invention, constituted by a projector provided with a triplespatial light modulator;

FIG. 18 is a schematic drawing of a fifth embodiment of the presentinvention, constituted by electronic photography apparatus which usesthe spatial light modulator of the present invention in place of apolygonal mirror;

FIG. 19 is a schematic drawing illustrating the construction of thesurroundings of a photosensitive drum of the electronic photographyapparatus shown in FIG. 18;

FIG. 20 is a schematic drawing of a sixth embodiment of the presentinvention in which the spatial light modulator is applied to an opticalcard as an optical switching device;

FIG. 21 is a schematic drawing of a seventh embodiment of the presentinvention, constituted by a spatial light modulator built into anexposure device;

FIG. 22 is a perspective exploded assembly view of a conventionalspatial light modulator;

FIG. 23 is a schematic drawing illustrating the spatial light modulationoperation of a conventional spatial light modulator;

FIGS. 24A to 24H show the fabrication process of the conventionalspatial light modulator shown in FIG. 22, and are each schematicdrawings illustrating steps of forming an intermediate layer on a lowerlayer;

FIGS. 25A to 25F are each schematic drawings illustrating steps offorming an upper layer on the intermediate layer obtained from theprocess shown in FIG. 24;

FIG. 26 is a perspective view of an eighth embodiment of a spatial lightmodulator according to the present invention;

FIGS. 27A and 27B are each schematic drawings illustrating the operationof the micromirror of the spatial light modulator shown in FIG. 26;

FIGS. 28A to 28I are each schematic drawings illustrating fabricationsteps of the spatial light modulator shown in FIG. 26; and

FIGS. 29A to 29J are each schematic drawings illustrating fabricationsteps of a ninth embodiment of the spatial light modulator according tothe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present spatial light modulator provided with a micromirror and themethod of manufacturing the same are now described in terms of a numberof preferred embodiments, with reference to the drawings.

First Embodiment Structural Description of the Spatial Light Modulator

FIG. 1 is a perspective exploded assembly view showing principalelements of a spatial light modulator of a first embodiment. In thisfigure, the spatial light modulator is basically constructed of asilicon mirror substrate 100, a glass electrode substrate 200, and acover glass 300.

The silicon mirror substrate 100 has a plurality of micromirrors 102arranged in a matrix. Of these micromirrors 102, a plurality arranged ina particular direction, for example the x-direction in the figure arecoupled by a torsion bar 104. Furthermore, surrounding the area in whichthe plurality of micromirrors 102 is arranged, a frame portion 106 isprovided. This frame portion 106 is coupled to both ends of each of thetorsion bars 104.

As shown in the enlargement in FIG. 2, these micromirrors 102 have slits108 formed in the periphery of the linkage portion with the torsion bars104. By the formation of these slits 108, the micromirrors 102 can beeasily driven in an inclining manner in the direction shown by an arrowin FIG. 2. Furthermore, the surface of the micromirrors 102 has formedon it a reflecting layer 102a. Thus, the micromirrors 102 can be drivenin an inclining manner by means of the driving operation describedbelow, whereby the reflected direction of light impinging on themicromirrors 102 can be changed. Additionally, by controlling the timefor which the light is reflected in a particular direction, the lightcan be modulated.

The dimensions of the micromirrors 102 and torsion bars 104 shown inFIGS. 2, 3A, and 3B are as follows.

    ______________________________________                                                      Minimum value                                                                           Maximum value                                         ______________________________________                                        Mirror width                                                                              W1       10 mm      100 mm                                          Mirror length L1  10 mm 100 mm                                                Mirror thickness T1 0.5 mm 5 mm                                               Torsion bar width W2   1 mm 5 mm                                              Torsion bar thickness T2 0.5 mm 5 mm                                          Torsion bar length L2   5 mm 20 mm                                            Cavity depth H 0.5 mm 20 mm                                                   Inclination angle q ±5° ±20°                            ______________________________________                                    

The glass electrode substrate 200 shown in FIG. 1 has a depression 202in the central region, and a rim portion 204 around the peripherythereof. One side of the rim portion 204 is cut away to form anelectrode removing opening 206, and on the outside of this electroderemoving opening 206 is formed an electrode removing strip 208continuous with the depression 202.

In the depression 202 of the glass electrode substrate 200, a pluralityof pillars 210 projecting upward from the depression 202 and having thesame height as the rim portion 204 are formed opposite to the torsionbars 104 located between two micromirrors 102 adjacent in thex-direction.

Furthermore, on the depression 202 and the electrode removing strip 208,a wiring pattern 212 is formed. This wiring pattern 212, as shown inFIG. 2, includes first and second address electrodes 214 and 216disposed opposite to the rear surfaces of the micromirrors 102 whichsandwich the torsion bar 104. Moreover, the first address electrodes 214extending in the y-direction are connected in common with first commonwiring lines 218. Similarly, the second address electrodes 216 extendingin the y-direction are connected in common with second common wiringlines 220.

On the glass electrode substrate 200 with the above construction, asshown in FIG. 1, a silicon mirror substrate 100 is anode-bonded. At thispoint, the end portions of the torsion bars 104 and the frame portion106 on the silicon mirror substrate 100 are bonded with the rim portion204 of the glass electrode substrate 200. Furthermore, intermediateportions of the torsion bars 104 of the silicon mirror substrate 100 andthe pillars 210 of the glass electrode substrate 200 are anode-bonded.Thereafter, the cover glass 300 is bonded on the frame portion 106 ofthe silicon mirror substrate 100. The end portions of the torsion bars104 coupled to the frame portion 106 are subjected to dicing at theposition of being cut away from the frame portion 106. Then the edgeportion including the rim portion 204 of the glass electrode substrate200 cut away to form the electrode removing opening 206 is hermeticallysealed with a sealing material to complete the first embodiment of thespatial light modulator. By further evacuating the interior of thespatial light modulator by any of various methods, the resistance of themicromirrors when driven is reduced, yielding a faster response and areduced power consumption.

Operating Principles of Spatial Light Modulator

When a micromirror 102 is driven to the "ON" orientation, simultaneouslya plurality of the micromirrors 102 aligned in the x-direction as shownin FIG. 1 will be electrically energized through the torsion bar 104. Onthe other hand, at the same time, the first and second addresselectrodes 214 and 216 as a combination are driven either in pointsequence or in line sequence, and by selecting the torsion bar 104 to beenergized in sequence in the y-direction of FIG. 1, the micromirrors 102arranged in a matrix can be driven to the "ON" orientation in apredetermined cycle.

On the other hand, to drive the micromirrors 102 to the "OFF"orientation, the polarity of the voltage applied to the first and secondaddress electrodes 214 and 216 is made the reverse of that applied fordriving to the "ON" orientation. In this way, the micromirrors 102 aredriven in an inclining manner in the direction opposite to that fordriving to the "ON" orientation.

Fabrication Process of Spatial Light Modulator

The fabrication process of the spatial light modulator of the firstembodiment is described with reference to FIGS. 4 and 5. First thefabrication process of the silicon mirror substrate 100 is described.

I. Fabrication Process of the Silicon Mirror Substrate 100

(1) Formation of Silicon Substrate Having Doped Layer

In this step, on one surface of a silicon substrate 110, shown in FIG.4A, a doped layer 112, shown in FIG. 4B, is formed. For this purpose alayer of for example a boron dopant is formed on the silicon substrate110 by spin coating. As this boron dopant may be used B₂ O₃ mixed withan organic solvent, such as the material PBF supplied by Tokyo OkaCompany. At this stage the thickness of the film of boron dopant may beadjusted by its dependence on the spin rotation conditions of thesilicon substrate 110 and the viscosity of the boron dopant.

In this embodiment, if the boron dopant has a viscosity of from 50 to100 cp, by adjusting the spin rotation conditions, the thickness of thefilm of boron dopant can be varied in the range 0.5 to 5.0 mm.

After spin coating with the boron dopant, heat may be applied in afurnace at 100 to 180° C. for 20 to 40 minutes, and the solvent of theboron dopant evaporated. In this embodiment, baking was carried out for30 minutes at 140° C. Further, by firing in an oxygen atmosphere for 1to 2 hours at 400 to 800° C., the binder can be removed. In thisembodiment, firing was carried out for 1 hour at 600° C. In thesubsequent step of thermal diffusion, thermal diffusion may be carriedout for 4 to 10 hours at 800 to 1200° C. in a nitrogen atmosphere. Inthis embodiment, thermal diffusion was carried out for 6 hours at 1100°C.

As a result, the boron (B) of the boron dopant was thermally diffusedinto the silicon substrate 110, and on the lower surface of the siliconsubstrate 110 the boron doped layer 112 shown in FIG. 4B was formed. Thethickness of the boron doped layer 112 can be adjusted in this thermaldiffusion step by varying the time and temperature conditions, and inthis embodiment, by carrying out thermal diffusion for 6 hours at 1000°C. a boron doped layer 112 of thickness 2 to 3 mm was formed.

In this case, the boron concentration in the boron doped layer 112 ispreferably at least 1×10¹⁸ atm/cm³. In this way, in the step of etchingthe silicon substrate 110 described below, the boron doped layer 112 canbe made to function as an etching stop layer.

To form the boron doped layer, it is also possible to use the borondiffusion plate method. In this case, the surface of the siliconsubstrate which is to be boron-doped is disposed in opposition to forexample a "Boron-purasu-ban" [plate] (trade name) produced by TekuneGurasu (Glass) Company. The spacing between these two should be 0.5 to4.0 mm, and preferably from 2.0 to 3.0 mm. With this spacing maintained,a thermal diffusion process is carried out in a thermal furnace at from800 to 1200° C., and for example at 1100° C., with a nitrogen flow rateof 3 to 8 liters/minute, and for example 6 liters/minute, for from 1 to6 hours, and for example for 2 hours.

As a further method, the ion implantation method can be used. In thiscase the acceleration energy is from 20 to 50 keV, and an optimum valueis 35 keV. The dose value which represents the number of acceleratedelectrons is appropriately from 2×10¹⁸ to 8×10¹⁸, and preferably4×10.sup.[/18/]. The beam current is appropriately from 1.5 to 4.6 mAand preferably 3.0 mA. Carrying out ion implantation under theseconditions resulted in a boron doped layer of between 0.5 and 4 mm.

(2) Thermal Oxidation Step

The silicon substrate 110 with the boron doped layer 112 formed thereon16 inserted in a thermal oxidation furnace, and as shown in FIG. 4C athermal oxidation film 114 is formed around the silicon substrate 110.In this embodiment, the thermal oxidation processing is carried out bythe wet oxidation method at a temperature of 1000° C. for 4 hours, and athermal oxidation film 114 of thickness 1 mm is formed. At this time theboron doped layer 112 is also thermally oxidized, and the thermaloxidation film 114 formed also on the surface thereof.

(3) Patterning Step

The silicon substrate 110 with the thermal oxidation film 114 issubjected to a photolithography process, and thus patterned as shown inFIG. 4D. As shown in this figure, on the front surface of the siliconsubstrate 110, a first mask 116 is patterned to form a window in thecentral region. On the underside of the silicon substrate 110 having theboron doped layer 112, a second mask 118 is patterned to form themicromirrors 102, torsion bars 104, frame portion 106, slits 108, andpossibly other elements shown in FIGS. 1 to 3. For this patterning, thefront surface and rear surface of the thermal oxidation film 114 areeach painted with resist, exposed, and developed. After the developmentstage, a buffered hydrofluoric acid solution is used to remove apredetermined portion of the thermal oxidation film (silicon oxide film)114. Thereafter, resist removal is carried out to complete thepatterning step. This resist removal can be carried out for example by amixture of sulfuric acid and aqueous hydrogen peroxide heated to 80° C.

(4) Step of Etching Silicon Substrate 110

As shown in FIG. 4E, using the first mask 116 formed on the uppersurface of the silicon substrate 110, the silicon substrate 110 isetched away. This etching step is carried out by wet etching of thesilicon substrate 110 with an aqueous solution of KOH of concentrationfrom 1 to 40% by weight. For the concentration of the aqueous solutionof KOH, a figure of approximately 10% by weight is ideal. The reactionin this etching step is shown by the following expression:

    Si+2KOH+H.sub.2 O →K.sub.2 SiO.sub.3 +2H.sub.2

Here, as shown in FIG. 4E, if the crystal orientation of the surface110a of the silicon substrate 110 is (100), then the side walls 110bformed by the etching process will be inclined at an angle of 55degrees. On the other hand, if the crystal orientation of the surface110a is (110), then substantially perpendicular side walls 110b can beobtained, and etching of high anisotropy realized. In this way, a largersurface area is made possible for the spatial light modulation.

In this case, the detection of etching completion can be achieved byobserving hydrogen bubbles generated by the reaction with the siliconsubstrate 110, and determining the point when bubbles stop beinggenerated as the completion point. Alternatively, by making the impurityconcentration of the boron doped layer 112 more than 1×10¹⁸ atm/cm³, theboron doped layer 112 can be used to function as an etching stop layer.

It should be noted that the etching fluid used in this step may be forexample an aqueous solution of TMAH (tetraethyl ammonium hydroxide), anaqueous solution of ethylenediamine-pyrocatechol-diazine (EPD), or anaqueous solution of hydrazine as an alternative to an aqueous solutionof KOH.

(5) Etching of Boron Doped Layer 112

As shown in FIG. 4F, using the second mask 118 formed on the thermaloxidation film 114 on the underside, the boron doped layer 112 isdry-etched.

This dry etching is preferably carried by the RIE (reactive ion etching)process, which has a fast etching rate and is adapted to volumeproduction. Here the processing gases used are CF₄ introduced at 30 to60 sccm and O₂ at 30 to 60 sccm, with a 13.56 MHz high frequency supplyof a power of 400 to 800 W, in particular as an optimum value set to 600W. The pressure within the chamber is preferably between 0.05 and 0.30Torr, and in this embodiment the optimum value of 0.15 Torr is adopted.In this embodiment, for etching a 2 mm boron doped layer, an etchingtime of 15 to 30 minutes is required.

By dry-etching the boron doped layer 112, the micromirrors 102, torsionbars 104, frame portion 106, and slits 108, shown in FIGS. 1 to 3 areformed.

(6) Step of Removing Thermal Oxidation Film 114

After the thermal oxidation film 114 has served as a masking materialfor the etching in step (5), the film 114 is removed. As a removalmethod may be used for example a buffered hydrofluoric acid solution, asdescribed in the patterning step (3). Alternatively, a dilute solutionof about 10% hydrofluoric acid may be used. In this embodiment of thisstep, as shown in FIG. 4G, the result is that the micromirrors 102,torsion bars 104, and other elements are supported by the frame portion106 formed on the boron doped layer 112.

(7) Vapor Deposition of Reflecting Layer 102a

On the surface of the micromirrors 102 formed on the boron doped layer112, a reflecting layer 102a of for example aluminum (Al) is formed byvapor deposition with a thickness of for example 0.2 to 2 mm. If thethickness of the reflecting layer 102a exceeds the upper limit, theinertial moment of the micromirrors 102 is increased. As a result, theresponse speed when driven is lowered, and the drive voltage for drivenin an inclining manner is increased. When the film thickness is belowthe lower limit, it is difficult to form a reflecting layer 102a ofuniform thickness to cover the whole boron doped layer 112.

At this time, the surface portions other than the micromirrors 102, inother words the torsion bars 104, may be masked to prevent the adhesionof aluminum, but in this embodiment a reflecting layer 102a may equallybe formed on the torsion bars 104. The top surface of the siliconsubstrate 110 remaining on the surface of the frame portion 106 willhereafter be a region subject to anode bonding with the glass electrodesubstrate 200, and it is important that the region is masked to preventthe adhesion of foreign objects which would impede the anode bonding ofthis portion.

It should be noted that the material of the reflecting layer 102a may beany material that reflects visible light with high efficiency, and forexample silver (Ag) may be used. The step of forming the reflectinglayer 102a is not restricted to a process of vapor deposition, and forexample sputtering may equally be used.

In the step of forming the reflecting layer 102a, since the boron dopedlayer 112 which forms the base is flat, the reflecting layer 102a canalso be formed thereon to be flat. In this way, light impinging on thereflecting layer 102a can be reflected with an angle of reflection equalto the angle of incidence. Furthermore, when the spatial light modulatoris used in the construction of a display device, the contrast can beincreased.

By the implementation of the above steps, the silicon mirror substrate100 shown in FIGS. 1 to 3 is completed. After this, as shown in FIG. 4I,the silicon mirror substrate 100, glass electrode substrate 200, andcover glass 300 are bonded. Before describing this bonding step, theprocess of fabricating the glass electrode substrate 200 is describedwith reference to FIG. 5.

II. Fabrication Process of Glass Electrode Substrate 200

As shown in FIG. 5A, a glass substrate 230 forming the base to the glasselectrode substrate 200 employs a glass substrate containing an alkalimetal such as sodium (Na) for the purposes of the anode bonding processdescribed below. As this type of glass substrate 230 may be used forexample sodium borosilicate glass such as Pyrex (trade name) from theCorning Company. In particular, in consideration of making thecoefficient of thermal expansion equal to that of silicon, since theglass substrate 230 should be heated in the anode bonding process,Corning#7740 (trade name) is optimum.

The fabrication process of the glass electrode substrate 200 using thisglass substrate 230 is now described.

(1) Patterning Step for Depression 202 and Other Elements

By painting a resist on the glass substrate 230 and carrying outexposure and development on the surface of the glass substrate 230 asshown in FIG. 5B, a resist pattern portion 232 is formed. The resistpattern portion 232 is formed in the positions corresponding to the rimportion 204 and the pillars 210.

(2) Etching to Form the Depression 202 and Other Elements

With the resist pattern portion 232 as a mask, wet etching of the glasssubstrate 230 is carried out with a hydrofluoric acid solution. In thisway, in addition to the depression 202 shown in FIG. 5C, the electroderemoving opening 206 and electrode removing strip 208 are alsosimultaneously formed. The depth of the depression 202 can be adjustedby varying the etching conditions, such as the etching time and thetemperature.

The depth of the depression 202 is an important element in determiningthe deflection angle of the micromirrors 102, and it is necessary toadjust the etching conditions as above so as to form the depression 202with a constant depth from lot to lot.

(3) Step of Resist Removal

This resist removal can be carried out using a mixture of sulfuric acidand aqueous hydrogen peroxide, and thereby as shown in FIG. 5D, theresist pattern portion 232 can be removed from the top surface of therim portion 204 formed on the glass substrate 230.

(4) Step of Forming Wiring Pattern 212

In order to form the wiring pattern 212 on the depression 202 andelectrode removing strip 208, first, as shown in FIG. 5F, over the wholesurface of the glass substrate 230 is formed an electrode film 234 of ametal such as aluminum (Al), silver (Ag), and gold (Au), or atransparent electrode material such as ITO. This electrode film 234 maybe formed by vapor deposition, sputtering, or ion plating. Thereafter,by carrying out a photolithography process on the electrode film 234, aresist pattern portion 236 is formed (see FIG. 5F). Next, using theresist pattern portion 236 as a mask, the electrode film 234 is etched.This etching can be carried out by wet etching.

Then as shown in FIG. 5G, by removing the resist pattern portion 236 onthe wiring pattern 212, the glass electrode substrate 200 is completed.The material used for removal should be selected appropriately dependingon the material of the electrode film 234. The material of the electrodefilm 234 may be ITO. In this case, a mixture of sulfuric acid andaqueous hydrogen peroxide cannot be used as removal agent, as it woulddissolve the ITO. In place of the mixture a removal agent based on anorganic solvent can be used.

III. Anode Bonding of Silicon Mirror Substrate 100 and Glass ElectrodeSubstrate 200

As shown in FIG. 6, the glass electrode substrate 200 is mounted on ahot plate 310 mounted on a positioning mechanism 314, and the siliconmirror substrate 100 is placed thereon. At this time, the first andsecond address electrodes 214 and 216 formed on the glass electrodesubstrate 200 and the micromirrors 102 formed on the silicon mirrorsubstrate 100 are positioned to face each other. This positioning iscarried out by observing from above with a microscope 316, while movingthe glass electrode substrate 200 two-dimensionally by means of thepositioning mechanism 314.

In the anode bonding process, if dirt or other foreign objects get inbetween the substrates 100 and 200, a short-circuit will occur betweenthe micromirrors 102 and the wiring pattern 212, and the product will bedefective. It is very important to carry out an inspection the presenceor absence of foreign objects before bonding the two substrates 100 and200. This inspection can be made before aligning the substrates 100 and200, or equally after the substrates are aligned if the wiring pattern212 is formed of a transparent electrode material such as ITO, byobservation with a microscope from below the glass electrode substrate200.

The silicon mirror substrate 100 and glass electrode substrate 200 areconnected to a DC power supply 312. The silicon mirror substrate 100 isconnected to the positive terminal of the DC power supply 312, and theglass electrode substrate 200 is connected to the negative terminal ofthe DC power supply 312. Then by means of the hot plate 310 the glasselectrode substrate 200 is heated to between 250 and 450° C., and forexample to 350° C., and 300 to 1000 V, for example 600 V, is appliedfrom the DC power supply 312 for 2 to 3 minutes.

In this way, by means of the heating by the hot plate 310, the Na+ ionsin the glass electrode substrate 200 become more motile. The motility ofthe Na+ ions imparts a negative charge to the bonding surface of theglass electrode substrate 200, and a positive charge to the bondingsurface of the silicon mirror substrate 100. As a result, a largeCoulomb force is generated between the two bonding surfaces, and at theinterface chemical bonding occurs, whereby the electrostatic bonding iscarried out. In this way, the silicon mirror substrate 100 and glasselectrode substrate 200 can be strongly bonded.

By means of this anode bonding, both ends of the torsion bars 104 andthe frame portion 106 of the silicon mirror substrate 100 are bonded tothe rim portion 204 of the glass electrode substrate 200. Thus, theintermediate portions of the torsion bars 104 of the silicon mirrorsubstrate 100 are bonded to the pillars 210 of the glass electrodesubstrate 200.

In this way, by the anode bonding of the silicon mirror substrate 100and the glass electrode substrate 200, a positive bond between the twois obtained, without the thickness of an adhesive film which followsfrom the use of an adhesive. Moreover, in contradistinction to the casein which an adhesive is used, there is none of the fluctuation inthickness of the adhesive film, and therefore the depth H of thedepression shown in FIG. 3B can be made substantially constant from lotto lot. This depth H of the depression determines the deflection angle qof the micromirrors 102, but by means of anode bonding the deflectionangle q can also be made substantially constant from lot to lot.

If the cover glass 300 is like the glass electrode substrate 200 made ofa glass substrate containing sodium, then the cover glass 300 cansimilarly be anode-bonded to the frame portion 106 of the silicon mirrorsubstrate 100. However, the bonding of the silicon mirror substrate 100and cover glass 300 does not require high precision, and therefore othermethods, such as an adhesive, may equally be used.

IV. Sealing, Dicing, and Wiring Processes

(1) Sealing Process

The electrode connecting opening 206 is hermetically sealed with asealing material. By doing this, in the dicing step (2) the ingress offoreign objects, water, or anything else to the space between thesilicon mirror substrate 100 and glass electrode substrate 200 isprevented. As a result, short-circuits occurring between themicromirrors 102 and the wiring pattern 212 are prevented, and the yieldof the process can be increased. Furthermore, in the dicing step (2), inthe two side walls of a spatial light modulator element 330, as shown inFIG. 7B, a very small gap 240 occurs with the sheet thickness of thetorsion bars 104, and therefore this very small gap 240 can similarlyalso be sealed.

(2) Dicing Process

As shown in FIG. 7A, a plurality of the spatial light modulator elements330 obtained as described above are fabricated simultaneously on asingle wafer 320 constituting the silicon substrate 110 shown in FIG.4A. Therefore, after the spatial light modulator element 330 iscompleted, the wafer 320 is diced to separate it into individual spatiallight modulator elements 330. By means of this dicing step, both ends ofthe torsion bars 104 coupled to the frame portion 106 are cut away, andthus function as independent electrodes separated in the y-direction inFIG. 1. A spatial light modulator element 330 separated by the dicingstep is as shown in FIG. 7B.

(3) Wiring Process

The separated spatial light modulator element 330 is fixed to asubstrate not shown in the drawings, and wiring carried out. As shown inFIG. 7B, the wiring process is carried out by connecting the wiringpattern 212 on the electrode connecting strip 208 and the ends of thetorsion bars 104 exposed in the side walls to a drive circuit not shownin the drawings. In this way, the spatial light modulator is completed.

Second Embodiment

A spatial light modulator capable of high density mounting is describedwith reference to FIGS. 8 to 10.

Overall Construction of High Density Mounting Spatial Light Modulator

The spatial light modulator of the second embodiment, as shown in FIGS.8 and 9, is basically constructed of a silicon mirror substrate 400, aglass electrode substrate 500, and a cover glass 600. The silicon mirrorsubstrate 400 and glass electrode substrate 500 are anode bonded in thesame way as in the first embodiment. The positional relationship of thesilicon mirror substrate 400 and the cover glass 600 has no effect onthe deflection angle of the micromirrors, and as in the firstembodiment, in place of anode bonding of the substrates 400 and 600,other bonding methods such as an adhesive may be used.

The silicon mirror substrate 400 has a plurality of micromirrors 402arranged in a matrix in the x- and y-directions as shown in FIG. 8,torsion bars 404 coupling a plurality of the micromirrors 402 arrangedin the x-direction, and a frame portion 406 coupling the ends of thetorsion bars 404. In the periphery of the linkage portion of themicromirrors 402 with the torsion bars 404 are formed slits 408. At oneend 404a each of the torsion bars 404 is directly coupled to the frameportion 406, while at the other end 404b it is coupled to the frameportion 406 through a mirror electrode 410 of large area.

One side of the frame portion 406 has formed therein an addresselectrode removing opening 412. Further, a divider 414 is provided todivide the region where the plurality of micromirrors 402 are arrangedfrom the address electrode connecting opening 412.

The silicon mirror substrate 400 is fabricated in quantity on a singlesilicon substrate, and during a dicing operation is separated from othersilicon mirror substrates at the four positions A to D shown in FIG. 8.

The glass electrode substrate 500 shown in FIG. 8 has a depression 502formed in the central region, and a rim portion 504 around the peripherythereof in the form of a rim. At positions corresponding to the divider414 of the silicon mirror substrate 400 the depression 502 is providedwith intermediate ribs 506. Additionally, at positions between twomicromirrors 402 adjacent in the x-direction in FIG. 8, and opposite thetorsion bars 404 the depression 502 is provided with pillars 508. In thedepression 502 of the glass electrode substrate 500, on both sides ofthe intermediate ribs 506 are formed a plurality of wiring patternportions 510 extending in the y-direction in FIG. 8. These wiringpattern portions 510 have first and second address electrodes 512 and514 opposing two regiin FIG. 8 are connected in common with secondcommon wiring lines 518. Furthermore, the end portions 516a and 518a ofthe first and second common wiring lines 516 and 518 are formeommonwiring lines 516. Similarly, the second address electrodes 514 extendingin the y-direction in FIG. 8 are connected in common with second commonwiring lines 518. Furthermore, the end portions 516a and 518a of thefirst and second common wiring lines 516 and 518 are formed to extend topositions beyond the intermediate ribs 506.

The glass electrode substrate 500 is also fabricated in quantity on asingle glass substrate, and is later subjected to a dicing operation toform a single spatial light modulator.

When the glass electrode substrate 500 is anode-bonded with the siliconmirror substrate 400, the end portions 516a and 518a of the first andsecond common wiring lines 516 and 518 are exposed through the addresselectrode connecting opening 412 formed as an opening in the siliconmirror substrate 400. When the silicon mirror substrate 400 and glasselectrode substrate 500 are anode-bonded, as in the first embodiment,the end portions 404a and 404b of the torsion bars 404 and the frameportion 406 on the silicon mirror substrate 400 are bonded with the rimportion 504 of the glass electrode substrate 500. Furthermore,intermediate portions of the torsion bars 404 of the silicon mirrorsubstrate 400 are anode-bonded to the pillars 508 of the glass electrodesubstrate 500. In this second embodiment moreover, the divider 414 ofthe silicon mirror substrate 400 is also bonded to the intermediate ribs506 of the glass electrode substrate 500.

The cover glass 600 shown in FIG. 8 has a depression 602 disposedopposite the disposition regions of the plurality of micromirrors 402formed on the silicon mirror substrate 400. Around the periphery of thisdepression 602 is formed a rim 604. In the fabrication process aplurality of the cover glasses 600 are fabricated on a single glasssubstrate and by dicing this glass substrate a single cover glass 600 asshown in FIG. 8 is constructed. One side 604a of the rim 604 is disposedto overlay one end 404a of each of the torsion bars 404 which have beensubject to dicing. Another side 604b of the rim 604 parallel to saidside 604a is disposed in a position to overlay the other end 404b ofeach of the torsion bars 404 which have been subject to dicing. As aresult, the mirror electrodes 410 of the silicon mirror substrate 400are exposed to the exterior. The remaining two sides 604c and 604d ofthe rim 604 at right angles to the above two sides 604a and 604b aredisposed in positions to overlay the frame portion 406 and divider 414of the silicon mirror substrate 400 respectively.

As a result, as shown in FIG. 9, the address electrode connectingopening 412 formed as an opening in the silicon mirror substrate 400 isexposed to the exterior, and wiring connections can be made to the firstand second common wiring lines 516 and 518 formed on the glass electrodesubstrate 500. After these wiring connections have been made, theaddress electrode connecting opening 412 is sealed with a sealingmaterial. Further, when the above-described dicing process is carriedout, in the regions where the ends 404a and 404b of the torsion bars 404are not present, as shown in FIG. 9, a gap 511 equivalent to thethickness of the torsion bars 404 between the rim portion 504 of theglass electrode substrate 500 and the rim 604 of the cover glass 600will be created. This gap 511 is also sealed with a sealing material inthe same way as described above.

Fabrication Process of Spatial Light Modulator

The fabrication process of a spatial light modulator of the secondembodiments described with reference to FIG. 10. The fabrication processof the glass electrode substrate 500 is substantially the same processas the process shown in FIG. 5. Since the cover glass 600 has thedepression 602 formed by etching, the process of FIGS. 5b to 5d may beapplied.

Next the points particular to the fabrication process of the spatiallight modulator of the second embodiment are described.

(1) Formation of Boron Doped Layer 422

In FIG. 10A, on one surface of a silicon substrate 420 is formed a borondoped layer 422. The formation of this boron doped layer 422 is carriedout in the same way as in the first embodiment.

(2) Anode Bonding of Substrates 400 and 500

A particular feature of the fabrication process of the spatial lightmodulator of the second embodiments that as shown in FIG. 10B, thesilicon substrate 420 or which the boron doped layer 422 has been formedis anode-bonded to the glass electrode substrate 500.

The anode bonding of the two substrates 400 and 500 is carried out inthe same way as in the first embodiment, using the apparatus shown inFIG. 6. Specifically, the glass electrode substrate 500 mounted on thehot plate 310 is heated to between 250 and 450° C., and for example to350° C. At the same time, the substrates 400 and 500 are connected to aDC power supply 312, while a negative voltage is applied to the glasselectrode substrate 500 connected to the negative terminal a positionvoltage is applied to and the silicon mirror substrate 400 connected tothe positive terminal. In this way, the Na+ ions in the glass electrodesubstrate 500 become motile, imparting a negative charge to the bondingsurface of the glass electrode substrate 500, and a positive charge tothe bonding surface of the silicon mirror substrate 400, whereby a largeCoulomb force is generated between the two bonding surfaces, andelectrostatic bonding is carried out.

The reason that the spatial light modulator of the second embodimentsadapted to high density mounting is that when the substrates 400 and 500are anode-bonded, the process can be completed without requiring thehigh-precision positioning of the micromirrors and wiring pattern whichin the first embodiment have already been formed. This high-precisionpositioning is required in the patterning of the reflecting layer 402ashown in FIG. 10E.

(3) Step of Wet-etching Silicon Substrate 420

As shown in FIG. 10C, the silicon substrate 420 formed above the borondoped layer 422 is etched completely. This wet etching uses the sameaqueous solution of KOH of a concentration of for example from 1 to 40%by weight, and follows the same etching reaction as shown in the firstembodiment to etch away the silicon. The etching completion can bedetected by observing the generation of hydrogen bubbles by the reactionwith the silicon substrate 420, and determining the point when bubblesstop being generated as the completion point. Alternatively, by makingthe impurity concentration of the boron doped layer 422 at least 1×10¹⁸atm/cm³, the boron doped layer 422 can be used to function as an etchingstop layer.

It should be noted that the solvent used for wet etching is notrestricted to an aqueous solution of KOH, and an aqueous solution ofTMAH, an aqueous solution of EPD, or an aqueous solution of hydrazinemay also be used.

(4) Vapor Deposition of Reflecting Layer 402a

As shown in FIG. 10D, a reflecting layer 402a is formed on the wholesurface of the boron doped layer 422. The reflecting layer 402a isformed of for example aluminum (Al) by vapor deposition. The thicknessof the reflecting layer 402a is, as in the first embodiment, from 0.2 to2 mm.

(5) Step of Patterning Reflecting Layer 402a

As shown in FIG. 10E, by a photolithography process, a resist pattern424 is formed on the reflecting layer 402a. This resist pattern 424corresponds to the micromirrors 402, torsion bars 404, frame portion406, slits 408, mirror electrodes 410, address electrode removingopening 412, and divider 414 formed on the silicon mirror substrate 400shown in FIG. 8.

When the reflecting layer 402a is formed of aluminum (Al), wet etchingcan be carried out using a mixture of phosphoric acid, acetic acid andnitric acid heated to 30° C., and immersing for 3 minutes. By this meansas shown in FIG. 10E, the reflecting layer 402a can be patterned.

At this time, if a double-sided alignment apparatus is used, thepatterning can be carried out with an accuracy of 0.5 mm. Specifically,from below in FIG. 10E, that is, from below the glass electrodesubstrate 500 the position of the wiring pattern portions 510 can berecognized through a microscope, and at the same time it is possible tomeasure through a microscope from above the silicon mirror substrate400. Thus, using the position of the wiring pattern portions 510 formedon the glass electrode substrate 500 as a datum, the exposure stage forforming the resist pattern 424 can be carried out. By this means,portions of the resist pattern 424 opposite to the micromirrors 402 andso forth can be formed in positions corresponding to the first andsecond address electrodes 512 and 514 of the wiring pattern portions510, and patterning can be accurate even when the mounting density isincreased.

Before the bonding of the two substrates 400 and 500, if the gaptherebetween should contain foreign objects, it is extremely difficultto remove these foreign objects. If foreign objects adhere to the wiringpattern portions 510 of the glass electrode substrate 500, when themicromirrors 402 are driven in an inclining manner, a short circuit willoccur, causing a defective product, and a reduction in yield.

In answer to this, in the second embodiment, before the anode bonding ofthe two substrates 400 and 500, light is shone from the transparentglass electrode substrate 500 side to check for the ingress of foreignobjects. At this point, it is preferable that the wiring patternportions 510 formed on the glass electrode substrate 500 are not of ametal such as aluminum or silver, but of a transparent electrodesubstance such as ITO. In this way, foreign objects adhering to the ITOsurface can also be detected.

(6) Dry Etching of Boron Doped Layer 422

For the etching of the boron doped layer 422, dry etching is moreappropriate than wet etching. The reason for this is that in the processof wet etching the boron doped layer 422, in the steps of etching,washing, and drying, foreign objects may be introduced from the outside,or drying marks may occur, whereas in dry etching these problems do notoccur.

For the dry etching of the boron doped layer 422 the resist pattern 424is of a mask material resistant to etching. This dry etching can becarried out under the same conditions as the process of FIG. 4F in thefirst embodiment.

(7) Step of Removing Resist Pattern 424

The resist pattern 424 can be removed, for example using a UV ashingdevice, in a dry process of ashing.

(8) Step of Bonding Cover Glass and Silicon Mirror Substrate

As in the first embodiment, the cover glass 600 does not require highprecision bonding, and therefore in place of anode bonding, an adhesivebonding method may equally be used.

(9) Sealing Process

To prevent the ingress of water or foreign objects in the dicingoperation, as shown in FIG. 9, the address electrode removing opening412 is sealed with a sealing material. Furthermore, the very small gap511 which is generated by the dicing process is also later sealed in thesame way.

(10) Dicing Process

In the process described above, a plurality of the substrates 400, 500,and 600 constituting a plurality of the spatial light modulator elementsare fabricated on single silicon and glass substrates. After the coverglass 600 bonding process is completed, each of the individual spatiallight modulator elements is separated by a dicing process. In thisdicing process, the substrates 400, 500, and 600 are each diced, at thefour positions A to D shown in FIG. 8. In this way, the torsion bars 404which were coupled together during the assembly process are eachseparated.

(11) Wiring Process

Finally, the mirror electrodes 410 and the ends 512a and 512b of thefirst and second address electrodes 512 and 514 of the separated spatiallight modulator are connected to a drive circuit not shown in thedrawings. In this way, the spatial light modulator is completed.

In the first and second embodiments described above, anode bonding isdescribed as an example method for bonding the silicon mirror substrateand the glass electrode substrate, but other methods can also be used inplace of this. As other examples direct bonding and diffusion bondingcan be cited.

Direct bonding can be used when the mirror substrate and electrodesubstrate are both silicon substrates. The silicon substrates are wellwashed, and after giving good wettability to the bonding surfaces of thesilicon substrates, for example an infrared microscope is used toprovisionally position the substrates at room temperature are. Next, theprovisionally positioned silicon substrates are heated to 800 to 1200°C., with an optimal temperature of 1100° C., and are heat-processed for1 to 4 hours in a nitrogen atmosphere, whereby the silicon substratescan be directly bonded together. In this case, when the wettability ofthe silicon substrates is good, the silicon surface elements are bondedas Si--O--H. When heat treatment is applied, by a reaction involving theloss of water, these are converted to Si--O--Si bonds, and the siliconsubstrates are bonded together.

An example of diffusion bonding is eutectic bonding, which can be usedwhen the electrode substrate bonding surface is of aluminum (Al) or gold(Au). These metals easily form an alloy at relatively low temperaturesfrom 300 to 350° C. If for example a gold film is formed on theelectrode substrate by sputtering, and this gold film is brought intocontact with the silicon mirror substrate, and subjected to heattreatment at 300 to 400° C. for 1 to 2 hours. The gold diffuses into thesilicon at the bonding interface, and the bonding interface disappearsto form a bond. This is not restricted to eutectic bonding, and adiffusion bonding may be used in which metals easily forming an alloyare formed on each of the electrode substrate and the silicon mirrorsubstrate. These metals are heated, and at the bonding interface onemetal diffuses into the other metal.

In this way, various materials may be used for the electrode substrate,according to the bonding method to be used. However, if the bondinginvolves the application of heat, the material should preferably have acoefficient of thermal expansion close to that of silicon.

Third Embodiment

The third embodiment described here is constructed so as positively toprevent a short-circuit between the micromirrors 402 and the first andsecond address electrodes 512 and 514. As shown in FIG. 11A, on thesurface of the micromirrors 402 facing the first and second addresselectrodes 512 and 514 is formed an insulating film 416. In this way,even if a foreign object 10 is present between the micromirror 402 andthe first and second address electrodes 512 and 514, by means of theinsulating film 416 a short-circuit between the micromirrors 402 and thefirst and second address electrodes 512 and 514 is prevented.

When the insulating film 416 is formed on the reverse side of themicromirrors 402, as shown in FIG. 11B, the first and second addresselectrodes 512 and 514 may also serve as stoppers for determining thedeflection angle of the micromirrors 402 when deflected. This insulatingfilm may, for example, in the case of the spatial light modulator shownin the first embodiment be formed by omission of the step of removingthe thermal oxidation film shown in FIG. 4G. In other words, the thermaloxidation film 114 formed in FIG. 4C can be used also as the insulatingfilm.

To prevent a short-circuit between the micromirrors 402 and the firstand second address electrodes 512 and 514, it is equally possible toprovide an insulation film on the surfaces of the first and secondaddress electrodes 512 and 514 film for example by using a sputteringdevice.

When the insulating film 416 is formed on the reverse side of themicromirrors 402, the surface of the first and second address electrodes512 and 514 may be made rough. By doing this, the contact area betweenthe micromirrors 402 and the first and second address electrodes 512 and514 can be reduced. If the surface is not made rough, a hot carriercharge occurs on the insulating film 416 formed on the micromirrors 402,and the micromirrors 402 sticks to the electrodes 512 and 514. Simply bymaking the surface of the electrodes 512 and 514 rough, this problem canbe avoided. This surface roughness can be provided by forming anelectrode surface with projections of height preferably at least 200Angstroms.

To provide the surface roughness on the surface of the first and secondaddress electrodes 512 and 514, for example when forming a film by thesputtering method, the conditions may be varied to use larger sputterparticles. Alternatively, if the electrodes 512 and 514 are formed byvapor deposition, the surface will be rough.

Alternatively, in an etching step for forming the depression 202 andother elements shown in FIG. 5C, by varying the etching conditions toprovide a rough etched surface, the ITO film can be formed thereon withthe surface roughness required.

To reduce the contact area between the micromirrors 402 and the firstand second address electrodes 512 and 514, as shown in FIG. 12, it isalso possible to form at each end of the micromirror 402 a micropyramid416a forming part of the insulating film 416 projecting downward. Toform such micropyramids 416a, the steps shown in FIGS. 13A to 13C may becarried out. First, as shown in FIG. 13A a first level oxide film 430 isformed on the insulating film 416. Next, on this first oxide film 430,and in positions corresponding to the ends of the micromirrors 402 areformed localized second oxide films 432.

Next, the first and second oxide films 430 and 432 are etched. By doingthis, as shown in FIG. 13B, if the etching rate is constant over thewhole surface, even after the first oxide film 430 in the central regionis completely removed, the peripheral portion of the first oxide film430 remains. Thereafter, as the etching proceeds, only the surface ofthe central region of the insulating film 416 is etched, and theinsulating film 416 at the extremities is not etched.

Then, as shown in FIG. 13C, by removing the oxide film remaining on theextremities, a micropyramid 416a formed on the insulating film 416 canbe formed at both ends of the micromirror 402.

To eliminate static charging of the insulating film 416 formed on thereverse surface of the micromirrors 402 by the first and second addresselectrodes 512 and 514, as shown in FIG. 14, an insulating stopper 530may be provided, projecting from the depression 502 of the glasselectrode substrate 500. In this way, even when the micromirrors 402 aredriven in an inclining manner, since they have a larger distance betweenthe insulating film 416 on the reverse surface and the first and secondaddress electrodes 512 and 514, the problem of sticking of themicromirrors 402 because of static charging is avoided.

Fourth Embodiment

An embodiment constituting a projector, using the spatial lightmodulator of the first embodiment or second embodiment is described,with reference to FIGS. 15 to 17.

FIG. 15 shows an embodiment constituting a projector using a singlespatial light modulator 700. As shown in this figure, white lightemitted by a projection lamp 702 passes through a condenser lens 704 tobe concentrated on a rotary color filter 706. This rotary color filter706 has filters of three colors: red ("R"), green ("G"), and blue ("B").By rotatably disposing the three color filters at the concentrationposition of the condenser lens 704, wavelengths of light of thesequentially changing colors are passed through the filters.

Light passing in sequence through the "R" "G" and "B" filters of therotary color filter 706 then goes via a condenser lens 708, a reflectingmirror 710, and a half prism 712, to impinge on the spatial lightmodulator 700. In this spatial light modulator 700, based on an imagesignal input from the outside, using the drive mechanism describedabove, the micromirrors are driven in an inclining manner in a scan inthe x-direction (horizontal direction) from one end in sequence, whilescanning sequentially in the y-direction (vertical direction), to causethe impinging light to be reflected from the individual micromirrorsdisposed in a matrix. By this means, for each pixel of the matrix inwhich the micromirrors are disposed, reflected light modulated accordingto gradation can be obtained.

This reflected light passes through the half prism 712 as parallellight, to impinge on a projection lens 714, and through the projectionlens 714 to be projected enlarged on a screen 716.

The spatial light modulator 700 of this embodiment has an opticalefficiency at least three times as high as a conventional liquid crystalpanel, which has a low optical efficiency because of the use ofpolarizing filters. The spatial light modulator 700 of this embodimentis thus able to display an image of adequate brightness on the screen716.

Moreover, the response time of the micromirrors at 20 ms, is vastlyfaster than the 30 ms of a conventional liquid crystal panel whichallows flickering on the screen to be prevented. Furthermore, in the useof a conventional liquid crystal panel, because of the low opticalefficiency three liquid crystal panels are required, corresponding to"R" "G" and "B". As a result, the alignment of the optical axes isextremely troublesome.

On the other hand, using the spatial light modulator 700 in thisembodiment an adequate brightness can be obtained with the singlespatial light modulator shown in FIG. 15, and alignment adjustment isextremely easy.

It should be noted that the spatial light modulator 700 can also be usedin a double or triple configuration as shown in FIG. 16 or 17. In thecase shown in FIG. 16 the rotary color filter 707 has "G" and "B"filters, and in the first stage of the two spatial light modulators 700a separating prism 717 is disposed to separate the light into twowavelength regions. On the other hand, in FIG. 17, no rotary colorfilter is used, and in the first stage of the three spatial lightmodulators 700 a separating prism 718 is disposed to separate the lightinto three wavelength regions. In this way, each of the spatial lightmodulators 700 may be shared by wavelengths of two colors, or may bedisposed in second or third stages for each wavelength independently. Bythis means, a brighter and clearer image can be projected on the screen716.

Fifth Embodiment

FIG. 18 shows an embodiment constituting a spatial light modulator 720of this embodiment applied to an electronic photography apparatus, suchas a laser printer. In this embodiment, the spatial light modulator 720is used in place of a conventional polygonal mirror. In this figure, thespatial light modulator 720 has a plurality of micromirrors 722 arrangedparallel to a beam of laser light emitted by a laser light source 724.

In FIG. 18, the micromirror 722 positioned at the rightmost end is shownas driven in the "ON" position, and in the direction of travel of thereflected light reflected when a micromirror 722 is in the "ON"position, is disposed a photosensitive drum 730. At this time, the othermicromirrors are in the "OFF" position, and do not block the passage oflaser light.

The photosensitive drum 730 is, as shown in FIG. 19, rotatable in forexample the clockwise direction. Around the periphery of thephotosensitive drum 730 are disposed an exposure lamp 732, a developingdevice 734, a transfer device 736, a cleaning device 738, and a chargeremoval device 740. By scanning the micromirrors 722 of the spatiallight modulator 720 shown in FIG. 18 sequentially from right to left,the surface charge on the photosensitive drum 730, which has alreadybeen charged to a particular level by the exposure lamp 732, is changedby the reflected light modulated by the micromirrors 722, to create alatent image. By means of the rotation of the photosensitive drum 730,toner is attached to the latent image by the developing device 734 todevelop the image, and the toner is transferred to a storage medium 744by the transfer device 736. The storage medium 744 is fixed by fixingrollers 742 disposed downstream, and is then ejected. After thecompletion of transfer, any remaining toner on the photosensitive drum730 is recovered by the cleaning device 738, then charge is removed bythe charge removal device 740 to restore the initial state.

In this way, according to this embodiment, the conventional polygonalmirror can be replaced by the spatial light modulator 720, and since thespatial light modulator 720 allows high density mounting and has a fastresponse speed, it is possible to record a high resolution image on thestorage medium 744.

Sixth Embodiment

The sixth embodiment is an embodiment of the spatial light modulator ofthis embodiment applied to for example an optical card capable ofoptical switching. As shown in FIG. 20, on an insulating substrate 750are provided a plurality of, for example twelve, induction coils 752-1to 752-12, capable of generating desirable induction voltages.

At an end of the insulating substrate 750 is disposed the spatial lightmodulator 760. The induction coils 752-1 to 752-12 are connected througha wiring pattern 754 to first and second address electrodescorresponding to six micromirrors provided in the spatial lightmodulator 760.

When this optical card is inserted into a reading device capable ofdetecting optical switching signals from the optical card, the twelveinduction coils 752-1 to 752-12 formed on the insulating substrate 750are disposed corresponding to twelve induction coils on the readingdevice. By energization thereof, induction voltages are generated ineach of the induction coils 752-1 to 752-12. Based on these inductionvoltages, the six micromirrors in the spatial light modulator 760 aredriven in an inclining manner, and a modulated optical switching signalcan be obtained by the reflection of light thereby.

Since the spatial light modulator 760 can be formed to be extremelysmall, it can easily be fitted into a portable card. Moreover, sincethis card is unrelated to the effect of magnetic fields, reading of thedata in the card for criminal purposes is prevented.

Seventh Embodiment

The seventh embodiment uses a spatial light modulator 780 of the presentinvention built into an exposure device, for writing a lot number andother information specific to the wafer on the surface of asemiconductor wafer 770 being exposed.

Opposing a wafer mounting stand 772 on which the wafer 770 is mounted isprovided a light source 774 for exposing the information on the wafer.Between the light source 774 and the wafer mounting stand 772, a reticle776 is provided to project light emitted by the light source 774 as apredetermined mask pattern image at reduced size on the wafer 770. Thespatial light modulator 780 built into this exposure device is disposedin a position to be impinged on by some of the light emitted by thelight source 774. Furthermore, when the micromirrors are driven to the"ON" position, light reflected therefrom falls on a predeterminedposition on the wafer 770.

The wavelength of the light source 774 of the exposure device will beshort: g-rays, i-rays, or excimer laser radiation as the element densityincreases. If light of these short wavelengths is modulated by anoptical switching device using liquid crystals, the enclosed liquidcrystals will soon deteriorate.

Because the present spatial light modulator 780 only reflects the shortwavelength light from micromirrors, it has adequate durability.

In this way, using the present spatial light modulator 780, the shortwavelength light from the light source 774 for exposure can also be usedfor recording ID information and the like on the wafer 770, and aseparate light source is unnecessary.

The present spatial light modulator is not limited to application to theabove-described devices, and can be applied to a range of devices inwhich light is modulated either at graduation, or simply in an on/offfashion. For example, by a construction so that the light reflected fromthe micromirrors of the spatial light modulator can be viewed directly,application to an advertising sign displaying characters including textand graphics, or to a watch display and so forth is possible.

Eighth Embodiment

Next, a spatial light modulator of an eighth embodiment according to thepresent invention, and the method of fabrication thereof are described.

FIG. 26 is a structural diagram of the present spatial light modulatorof the eighth embodiment. In this figure, the spatial light modulatorhas a three-layer construction which can be divided broadly into asilicon circuit substrate 910 on which a drive circuit including an SRAMis formed, a silicon mirror substrate 920 on which micromirrors 930 areformed, and a glass substrate 940.

The plurality of micromirrors 930 formed on the silicon mirror substrate920 are coupled in one direction by torsion bars 932, and both ends ofthese torsion bars 932 are fixed to a frame portion 934. The frameportion 934 is bonded between the silicon circuit substrate 910 and theglass substrate 940. Furthermore, the frame portion 934 is an importantcomponent in determining the gap between the micromirrors 930 andelectrodes 911 on the silicon circuit substrate 910. In other words, thedrive torque and deflection angle of the micromirrors 930 can beadjusted by the thickness of the frame portion 934.

The glass substrate 940 has a central depression 942, a rim portion 944around the periphery thereof, and pillars 946 projecting from thedepression 942. These pillars 946 abut a torsion bar 932 between twomicromirrors 930 adjacent in one direction.

The depression 942, rim portion 944, and pillars 946 are formed byetching of the glass substrate 940.

Further, on the surface of the micromirrors 930 is formed a reflectivefilm 933. The material of the reflective film 933 may appropriately bealuminum or silver with a high reflectivity of visible wavelengths oflight.

Individual micromirrors 930 are formed communicatingly from silicon inwhich are diffused impurities of extremely high conductivity, andthrough the frame portion 934 are coupled to the drive circuit of thesilicon circuit substrate 910. Based on driving by the drive circuit, anelectrical field occurs between the surface electrodes 911 and themicromirrors 930, so that the positions of the micromirrors 930 can bevaried.

By varying the positions of the micromirrors 930, two reflection modescan be obtained: one, as shown in FIG. 27A, in which a light ray 950impinging diagonally from the upper right is reflected back diagonallyto the upper right as a light ray 951, and the other, as shown in FIG.27B, in which the light ray 950 is reflected diagonally to the lowerleft as a light ray 952. Then by providing a projection lens in theonward path of the reflected light shown in FIG. 27B, for example aprojector can be constructed.

Next the method of fabrication of this spatial light modulator isdescribed.

FIGS. 28A to 28I show steps in the fabrication process of the spatiallight modulator of the eighth embodiment according to the presentinvention.

First a glass substrate 940 whose principal component is borosilicate ispainted with a resist, and a process of photolithography is applied tothe resist to carry out patterning. Using the resist patterned in thisway, the glass substrate 940 is wet-etched, whereby as shown in FIG. 28Athe depression 942, rim portion 944, and pillars 946 are formed. Herethe depth of this etching should not restrict the deflection angle ofthe micromirrors 930, and should therefore be more than the deflectionhalf-amplitude (W/2) of the micromirrors 930 shown in FIG. 27A.

The deflection angle q shown in FIG. 27A can be found from the followingexpression:

    sin q=(amplitude W/micromirror side length)                (1)

Therefore, the deflection half-amplitude (W/2) of the micromirrors 930is given by:

    Deflection half-amplitude (W/2)=micromirror side length×sin q/2(2)

And it is necessary for the height of the pillars 946 to be more thanthis deflection half-amplitude (W/2).

The dimensions of a sample spatial light modulator are:

Micromirror side length: 16 mm

Micromirror thickness: 1 mm

Deflection angle: 10 degrees

From expression (2), the deflection half-amplitude (W/2) is 1.4 mm. Inthis case the pillars 946 are made with a height of 2 mm. The height ofthe pillars 946 is obtained by controlling the etching time.

Next, a silicon substrate 921 for forming the micromirrors 930, torsionbars 932, and frame portion 934, is doped with impurities.

These impurities provide an etch stop effect in the etching of thesilicon by an alkaline aqueous solution, and the etch stop effect is anecessary technique in the fabrication of the micromirrors 930 whichrequire an extremely thin and accurate substrate.

The diffusion depth of the impurities is an important factor indetermining the thickness of the micromirrors 930.

Specifically, the impurities consist of boron, and as a boron dopant amixture of the boron compound B₂ O₃ with an organic solvent is used. Asthis boron dopant for example the material PBF supplied by Tokyo OkaCompany may be used. The boron dopant is spin-coated on the siliconsubstrate 921, and the film thickness of boron dopant may be adjusted byits dependence on the spin rotation conditions of the silicon substrate921 and the viscosity of the boron dopant.

In this embodiment, if the boron dopant has a viscosity of from 50 to100 cp, by adjusting the spin rotation conditions, the film thickness ofboron dopant can be varied in the range 0.5 to 5.0 mm.

After spin coating with the boron dopant, the silicon substrate 921 maybe heated in a furnace at 100 to 180° C. for 20 to 40 minutes, and thesolvent of the boron dopant evaporated. In this embodiment, the siliconsubstrate 921 is baked for 30 minutes at 140° C. Further, by firing thesilicon substrate 921 in an oxygen atmosphere for 1 to 2 hours at 400 to800° C., the binder is removed. In this embodiment, the siliconsubstrate 921 is fired for 1 hour at 600° C. In the subsequent step ofthermal diffusion, thermal diffusion may be carried out for 4 to 10hours at 800 to 1200° C. in a nitrogen atmosphere. In this embodiment,thermal diffusion is carried out for 5 hours at 1100° C.

As a result, as shown in FIG. 28B, the boron of the boron dopant isthermally diffused into the silicon substrate 921, and a 1 mm borondoped layer 929 is formed.

The thickness of the boron doped layer 929 can be adjusted in thisthermal diffusion step by varying the temperature and processing time.

Next, by thermal oxidation of the silicon substrate 921, a 0.2 mm oxidefilm is formed, and in the positions where a flame portion 934 are to beformed a photolithography process and oxide film etching step are usedto carry out patterning, and as shown in FIG. 28C, a glass supportpattern 927 not covered by the oxide film 928 is obtained.

Boron doping is carried out once more, and boron diffused more deeplyinto the glass support pattern 927 only. At this time, the oxide film928 acts as a barrier to the diffusing substance, and where the oxidefilm 928 is present, diffusion does not occur even if the diffusingsubstance is present thereon. For this reason, the oxide film 928 actsas a mask to allow selective boron doping. In this way, as shown in FIG.28D, a frame portion doped layer 926 and the boron doped layer 929 canbe formed.

At this time, the following relation holds between the frame portiondoped layer 926 and boron doped layer 929:

    Micromirror 930 deflection half-amplitude (W/2)=(frame portion doped layer 926)-(boron doped layer 929)

And if the deflection half-amplitude (W/2) requires 1.4 mm, thediffusion thickness of the frame portion doped layer 926 should be 2.4mm from the above expression.

After formation of the frame portion doped layer 926, the oxide film 928is removed with hydrofluoric acid, and to improve the opticalreflectivity of the boron doped surface, an optical reflecting film 933is formed. In concrete terms, the film is formed by vapor deposition ofaluminum. This state is shown in FIG. 28E.

Next, the etched surface of the glass substrate 940 fabricated in thestep shown in FIG. 28A, and the silicon substrate 921 processed asdescribed above, are bonded by the anode bonding method.

Specifically, the bonding conditions are that the two substrates 921 and940 placed on a hot plate heated to 250° C. are bonded by applying 500 Vbetween them. The state after bonding is shown in FIG. 28F.

The silicon substrate 921 and glass substrate 940 bonded together areimmersed in an aqueous solution of KOH heated to 80° C., and the siliconsubstrate 921 is subjected to wet etching. As a result, as shown in FIG.28G, a thin silicon film 925 remained on the underside of the opticalreflecting film 933.

Thereafter, using a dicing apparatus, the silicon mirror substrate 920and glass substrate 940 bonded together are cut to a specified size.

Next a resist pattern is formed on the thin silicon film 925 of thesilicon mirror substrate 920 and glass substrate 940 bonded together,and the thin silicon film 925 is etched by dry etching. Thereafter, adry resist removal process was carried out. As a result, as shown inFIG. 28H, the micromirrors 930, torsion bars 932, and frame portion 934are formed. The reason for using dry processes for the etching step andresist removal step is to avoid the problems with wet processing such asadhering debris remaining after resist removal, resist remaining onoverhang portions, and drying marks.

Next, the silicon circuit substrate 910 provided with the drive circuitfor the spatial light modulator and the bonded glass-silicon unitprocessed in FIG. 28H are bonded, and the structure shown in FIG. 28I isobtained. At this point, the frame portion 934 is accurately positionedso as to be coupled to the appropriate position of the silicon drivecircuit of the silicon circuit substrate 910 before bonding.

In this bonding, pad portions of the drive circuit are gold plated, andthe phenomenon is utilized that this gold plating and the silicon of thesilicon circuit substrate 910 form eutectic crystals at approximately320° C., to bond. By using this bonding method, variations in gapdimensions due to uneven spreading of an adhesive or the like areavoided, and a high gap accuracy is obtained. In place of this, it isalso possible to use a spin coating of a conductive adhesive, and toachieve the bonding by means of the evenly spread conductive adhesive.

Thereafter, by dicing along the broken line shown in FIG. 28I,individual spatial light modulator elements are separated.

Finally, appropriate wiring of the silicon circuit substrate 910provided with the drive circuit is carried out, to complete thefabrication of the spatial light modulator.

Ninth Embodiment

In the eighth embodiment described above, the micromirrors arefabricated from silicon, but as described in this ninth embodiment, ametal film can also be used. FIGS. 29A to 29J show the fabricationprocess of this ninth embodiment of the spatial light modulator. Thedescription here does not include detailed conditions of the steps inthe process, but the corresponding conditions described in the first andsecond embodiments may be used.

First, as shown in FIG. 29A, on a glass substrate 1000 a first resistpattern 1002 is formed. Next, as shown in FIG. 29B, a first metal film1004 of aluminum or the like is formed by vapor deposition, sputtering,or another method on the surface of the glass substrate 1000 and resistpattern 1002. Then as shown in FIG. 29C, in order to etch the firstmetal film 1004 a second resist pattern 1006 is formed on the firstmetal film 1004. It should be noted that in the ninth embodiment, firstto fourth resist patterns 1002, 1006, 1016, 1020 are used, and for thesecond resist pattern 1006 only, post-baking is not carried out afterdevelopment.

Next, as shown in FIG. 29D, using the second resist pattern 1006 as amask, the first metal film 1004 is etched. At this point, micromirrors1008, torsion bars 1010, and pillars 1012 are formed. Furthermore, onthe periphery of the glass substrate 1000 , a first rim 1014 is formed.This first rim 1014 supports both ends of the torsion bars 1010.

The remaining second resist pattern 1006 is exposed, for example byirradiated ultraviolet light, and thereafter developed. At the time ofthis development, only the second resist pattern 1006, which has notbeen subject to post-baking, is removed as shown in FIG. 29E.

Next, in order to extend the first rim 1014 to form a second rim 1022(see FIG. 29H), as shown in FIG. 29F, a third resist pattern 1016 isformed.

After this, as shown in FIG. 29G, a second metal film 1018 of aluminumor the like is formed by vapor deposition or sputtering on the first rim1014 and third resist pattern 1016. Then on the second metal film 1018,a fourth resist pattern 1020 is formed on the second metal film 1018, toact as an etching mask for the second metal film 1018.

Then, as shown in FIG. 29H, the second metal film 1018 is etched,whereby the first rim 1014 is extended to form a second rim 1022.

All resist patterns, 1002, 1016 and 1020, remaining on the glasssubstrate 1000 are removed, whereby as shown in FIG. 29I the glasssubstrate 1000 provided with the micromirrors 1008, torsion bars 1010,pillars 1012, and second rim 1022, all formed of metal, is completed.

On the other hand, on the surface of a silicon circuit board 1030provided with an SRAM, as shown in FIG. 29J, are formed drive electrodes1032 for driving the micromirrors 1008, gold electrodes 1034 to bebonded to the second rim 1022, and terminals 1036. The gold electrodes1034 on this silicon circuit board 1030 and the second rim 1022 formedon the glass substrate 1000 are bonded by the Au--Al diffusion bondingmethod. The temperature for this bonding is set to be 100 to 200° C.,and the bonding time at least 1 minute. In place of this diffusionbonding, bonding with a uniformly spread conductive adhesive may also beused.

Finally, wiring to the terminals 1036 of the silicon circuit board 1030is carried out to complete the spatial light modulator.

While the invention has been described in conjunction with severalspecific embodiments, it is evident to those skilled in the art thatmany further alternatives, modifications and variations will be apparentin light of the foregoing description. Thus, the invention describedherein is intended to embrace all such alternatives, modifications,applications and variations as may fall within the spirit and scope ofthe appended claims.

What is claimed is:
 1. A method of fabricating a spatial lightmodulator, comprising the steps of:(a) forming a conductive siliconmirror substrate having:a plurality of micromirrors arranged in one of aline and matrix; at least one torsion bar coupling said micromirrors inone direction; and reflective layers formed at least on one surface ofsaid micromirrors; (b) forming an electrode substrate having:adepression in a central region; a rim around the periphery thereof; setsof electrodes having conducting layers disposed within said depressionin positions corresponding to said micromirrors for driving saidmicromirrors in an inclining manner by means of a coulomb force; andpillars projecting from said depression in positions corresponding to aninterval between two of said micromirrors adjacent in said onedirection; and (c) positioning at least intermediate portions of said atleast one torsion bar on said silicon mirror substrate opposite to saidpillars of said electrode substrate, and bonding said silicon mirrorsubstrate to said electrode substrate.
 2. The method according to claim1, wherein said step (a) comprises the steps of:doping a siliconsubstrate with impurities to form a doped layer; patterning a first maskfor forming a window on one surface of said silicon substrate and asecond mask for forming said plurality of micromirrors and said at leastone torsion bar on the other surface of said silicon substrate; etchingsaid silicon substrate until said doped layer is exposed using saidfirst mask; a step of etching said doped layer using said second mask;removing said first and second masks and forming said plurality ofmicromirrors and said at least one torsion bar from said doped layer;and forming said reflective layers on one surface of said micromirrorsof said doped layer.
 3. The method according to claim 1, wherein saidstep (a) includes a step of forming a frame portion coupled with bothends of said at least one torsion bar and surrounds a region in whichsaid micromirrors are disposed on said silicon mirror substrate;saidstep (c) includes a step of bonding said frame portion and both ends ofsaid torsion bar with said rim of said electrode substrate; and aftersaid step (c) a step is provided to cut away both ends of said at leastone torsion bar from said frame portion of said silicon mirrorsubstrate.
 4. The method according to claim 3, wherein said step (a)comprises the steps of:doping a silicon substrate with impurities toform a doped layer; patterning a first mask for forming a window on onesurface of said silicon substrate and a second mask for forming saidplurality of micromirrors, said at least one torsion bar, and said frameportion on the other surface of said silicon substrate; etching saidsilicon substrate until said doped layer is exposed using said firstmask; etching said doped layer using said second mask; removing saidfirst and second masks, and forming said micromirrors, said at least onetorsion bar, and said frame portion from said doped layer; and formingsaid reflective layers on one surface of said micromirrors of said dopedlayer.
 5. The method according to claim 4, wherein the impurityconcentration of said doped layer is at least 1×10¹⁸ atm/cm³ ; andsaiddoped layer functions as an etching stop layer when said siliconsubstrate is etched.
 6. The method according to claim 1, wherein saidelectrode substrate is formed from a glass electrode substrate includingan alkali metal; andsaid step of bonding uses an anode bonding method.7. The method according to claim 6, wherein said glass electrodesubstrate is formed by the steps of:masking positions corresponding tosaid rim and said pillars and etching a glass substrate including analkalimetal to form said depression of a predetermined depth; andforming said sets of electrodes on the base of said depression.
 8. Themethod according to claim 1, further comprising a step of bonding atransparent cover plate on said silicon mirror substrate so as to coversaid silicon mirror substrate and in a position of non-interference withsaid micromirrors driven in an inclining manner.
 9. A method offabricating a spatial light modulator having a plurality of micromirrorsarranged in one of a line and matrix comprising the steps of:(a) formingan electrode substrate having:a depression in a central region; a rimaround the periphery thereof; sets of electrodes having conductinglayers disposed within said depression in positions corresponding tosaid micromirrors for driving said micromirrors in an inclining mannerby means of a coulomb force; and pillars projecting from said depressionin positions corresponding to an interval between two of saidmicromirrors adjacent in said one direction; (b) bonding a siliconsubstrate having a doped layer doped with impurities on one surfacethereof to said electrode substrate so that at least said pillars ofsaid electrode substrate are opposite to said doped layer; (c) etchingand removing said silicon substrate so that said doped layer is left;(d) forming a reflective layer on a surface of said doped layer; and (e)etching said doped layer and said reflective layer to form a siliconmirror substrate from said doped layer and reflective layer, saidetching step having the steps of:forming said plurality of micromirrorsin positions opposite said sets of electrodes, and forming at least onetorsion bar coupled with said micromirrors in said one direction, saidat least one torsion bar being bonded to said pillars at positionsbetween two of said micromirrors adjacent in said one direction and toone of said rim and pillers at both ends.
 10. The method according toclaim 9, wherein said step (e) includes a step of forming a frameportion on said silicon mirror substrate said frame portion couplingwith both ends of said at least one torsion bar, and being bonded tosaid rim of said electrode substrate in a position surrounding a regionin which said micromirrors are disposed; andafter said step (e) a stepis provided to cut away both ends of said at least one torsion bar fromsaid frame portion of said silicon mirror substrate.
 11. The methodaccording to claim 9, wherein the impurity concentration of said dopedlayer is at least 1×10¹⁸ atm/cm³ ; andsaid doped layer functions as anetching stop layer when said silicon substrate is etching.
 12. Themethod according to claim 9, wherein said electrode substrate is formedof a glass electrode substrate having optical transparency; andtheposition of the pattern of said sets of electrodes is observed from theside of said glass electrode substrate, and the mask pattern is alignedfor the etching of step (e) using the pattern position as a reference.13. The method according to claim 9, wherein said electrode substrate isformed from a glass electrode substrate including an alkali metal;andsaid step of bonding uses an anode bonding method.
 14. The methodaccording to claim 13, wherein said glass electrode substrate is formedby the steps of:masking positions corresponding to said rim and saidpillars and etching a glass substrate to form said depression of apredetermined depth; and forming said sets of electrodes on the base ofsaid depression.
 15. The method according to claim 12, wherein said setsof electrodes are formed as sets of transparent electrodes, said methodfurther comprising a step of inspecting the presence of foreign objectsbetween said glass electrode substrate and said silicon mirror substratefrom the side of said glass electrode substrate.
 16. The methodaccording to claim 9, further comprising a step of forming an insulatingfilm on a surface where said micromirrors is opposite said sets ofelectrodes.
 17. The method according to claim 16, wherein a surface ofsaid sets of electrodes where said electrodes are opposite saidinsulating film formed on said micromirrors is formed to be rough. 18.The method according to claim 16, further comprising a step of formingan insulating projection on said insulating film and at a positiondisplaced from said at least one torsion bar.
 19. The method accordingto claim 16, further comprising a step of forming an insulating stopperprojecting from the base of said depression of said electrode substrateto a height less than the height of said rim and said pillars, saidinsulating stopper abutting said micromirrors when driven in aninclining manner to determine the deflection angle.
 20. The methodaccording to claim 9, further comprising a step of bonding a transparentcover plate on said silicon mirror substrate so as to cover said siliconmirror substrate and in a position of non-interference with saidmicromirrors driven in an inclining manner.
 21. A method of fabricatinga spatial light modulator, comprising the steps of:(a) forming on aglass substrate a depression in a central region thereof, a rimsurrounding said depression, and pillars formed to project from saiddepression; (b) diffusing impurities into one surface of a siliconsubstrate to a predetermined depth; (c) further diffusing impuritiesinto a predetermined portion of said one surface of said siliconsubstrate to a predetermined depth to form an impurity diffusionsurface; (d) forming an optically reflective film on said impuritydiffusion surface of said silicon substrate; (e) bonding said impuritydiffusion surface and said rim of said glass substrate, to form asilicon-glass bonded substrate; (f) wet-etching said silicon-glassbonded substrate to make said silicon substrate into a thin film; (g)dry-etching said silicon substrate of said thin film to form a pluralityof micromirrors, a torsion bar coupling and supporting the same, and aframe portion fixing both ends of said torsion bar; and (h) bonding tosaid frame portion of said silicon-glass bonded substrate a siliconcircuit substrate provided with a plurality of pairs of electrodes fordriving said plurality of micromirrors, and circuit elements applying adrive voltage to said electrodes.
 22. The method according to claim 21,wherein in said step (h) said silicon circuit substrate and said frameportion of said silicon-glass bonded substrate are subjected to eutecticbonding.
 23. The method according to claim 21, wherein in said step (h)said silicon circuit substrate and said frame portion of saidsilicon-glass bonded substrate are bonded using a conductive adhesive.24. The method according to claim 21, wherein in said step (c) boron isused as an impurity element diffused into said silicon substrate.
 25. Amethod of fabricating a spatial light modulator comprising the stepsof:(a) forming a first resist pattern on a glass substrate to formpillars in a central region thereof and a first rim on the peripherythereof; (b) forming a first metal film on said glass substrate and saidfirst resist pattern; (c) forming on said first metal film a secondresist pattern to form micromirrors and a torsion bar; (d) etching saidfirst metal film using said second resist pattern; (e) removing saidsecond resist pattern; (f) forming a third resist pattern in a regionexcluding a surface of said first rim; (g) forming a second metal filmon said first rim and said third resist pattern; (h) forming a fourthresist pattern on said second metal film and in position opposite saidfirst rim; (i) etching said second metal film using said fourth resistpattern, and extending said first rim to form a second rim; (j) removingsaid first, third, and fourth resist patterns; and (k) bonding a siliconcircuit substrate provided with circuit elements for driving saidmicromirrors and said second rim of said glass substrate.
 26. The methodaccording to claim 25, wherein in said step (k) said silicon circuitsubstrate and said second rim of said glass substrate are subjected todiffusion bonding.
 27. The method according to claim 25, wherein in saidstep (k) said silicon circuit substrate and said second rim of saidglass substrate are bonded using a conductive adhesive.